The production of sintered parts from "green" bodies is well known in the art. Generally, the green body is formed by filling a die with a powder/binder mixture and compacting the mixture under pressure to produce the green body. The green body, a self supporting structure, is then removed from the die and sintered. During the sintering process, the binder is volatilized and burned out. However, removal of the binder can cause the product to crack, shrink and/or become distorted.
The injection molding of parts from metallic and/or ceramic powders has been a particularly troublesome process. Suitable injection molding compositions must be those which are capable of transforming from a highly fluid state (necessary for the injection step to proceed) to a solid state having a high green strength (necessary for subsequent handling). In order to meet these requirements, the prior art molding compositions comprise a relatively high percentage of a low melting point binder, such as, wax. However, such systems exhibit a number of problems in forming parts, especially parts of complex shapes.
More specifically, waxes are commonly employed as binders because they fulfill the rheological requirements of high fluidity at moderately elevated temperatures and substantial rigidity at temperature below about 15.degree. C. Wax formulations normally comprise between about 10% and about 20% wax by weight of the formula. During the firing process, wax is initially removed from the body in liquid form. During this initial step of the firing process, the green body may disintegrate or become distorted. Consequently, it is often necessary to preserve the shape of the green body by immersing it in an absorbent refractory powder (capable of absorbing the liquid wax). Notwithstanding the use of the supporting powder to retain the shape of the body, products produced from these systems ordinarily shrink and are relatively porous. Moreover, the formation of complex shapes from wax based systems is even more difficult because it requires, in most instances, detailed firing schedules which may encompass several days in an attempt to avoid the development of cracks in the part.
More recently, the use of methylcellulose polymers as binders in the manufacture of parts from metals or ceramic powders has been disclosed. Sarkar et al., in "Methylcellulose Polymers as Multi Functinal Processing Aides in Ceramics", Ceramics Bulletin, Vol. 62, No. 11, pp. 1281-1288 (1983), disclose the extrusion of honeycombed ceramic structures employing methylcellulose and discuss the "gellation" of methylcellulose-containing solutions at elevated temperatures. In addition, U.S. Pat. No, 4,113,480 discloses the use of methylcellulose or other plastic media (e.g., polyvinyl alcohol) in forming injection molded metallic parts.
We have discovered novel molding compositions useful in forming ceramic and/or metal parts which not only reduce the firing times and regimens for such parts, but also allow for the production of complex shapes without the attendant shrinkage and cracking problems associated with the prior art products.